<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>Document</title>
	<style>
		.wrap{
			height: 80px;
			background-color:blue;
			width：100px;<!--扩展性不好，没法设置宽度-->
		}
		.wrap div{
			width: 50px;
			height: 50px;
			float: left;
			background-color: red;
		}
	</style>
</head>
<body>
	<div class="wrap">4
		<div>1</div>
		<div>2</div>
		<div>3</div>
	</div>
</body>
</html>